In U.S. Pat. No. 5,419,171, Bumgarner (Boeing) describes isostatic bulge forming of tube stock. Many metal ducts and tubes having irregular bends, bulges, or indentations are in commercial aircraft, for example, for air conditioning or heating ducts or engine bleed air systems. An environment requiring one of the most convoluted metal tubes is the duct that carries hot exhaust gases from the jet engines.
Ducts that are relatively straight and that do not have harsh or abrupt bulges or indentations are commonly shaped using conventional bulge forming methods, including hydroforming. U.S. Pat. 2,372,917 describes a conventional method of bulge forming a tube using a split female tool. A liquid is pressurized in the interior of the tube while pressure is vented from the tool cavities. German Patent 24 42 801 describes another bulge forming method featuring an intermediate fluid filled chamber to accomplish pressurization of the forming liquid. U.S. Pat. No. 3,564,886 shows another, similar bulge-forming method using a vented tool. Finally, U.S. Pat. Nos. 3,359,624 and 3,462,821 describe pipe forming using conventional bulge forming techniques. I incorporate all these patents by reference.
A conventional bulge forming apparatus consists of an upper platen and lower platen. A jig collar holds two, matching die halves together around the tube stock blank. The blank is formed to expand outwardly to conform with and to match the interior contour of the dies. The blank is held firmly with a bottom and top piston. An incompressible fluid is fed into the blank through an inlet in the bottom piston. Air is simultaneously evacuated through an outlet in the top piston. When all the air is gone, a valve on the outlet is closed and pressure is applied to the fluid causing the blank to bulge into the dies.
This conventional method has many limitations. It requires a high tonnage press and high strength machined tools that will withstand the application of hydraulic pressures up to 20,000 psi. While the method may be cost effective to make thousands of the same parts, it is expensive to provide a high tonnage press and such tooling for parts making only a few parts with short production runs. Moreover, the process is essentially limited to the manufacture of rather straight tubes of rather short length and small interior volume. In addition, it requires substantial scrap from the trimming of the blank above and below the bulge formed section of pipe.
Most part runs of large tubes and ducts make a small number of parts measured in the hundreds per year at current record production rates for commercial transport aircraft. Forming such tubes has commonly been done by hammer forming small segments and then welding the segments together. While tooling costs for this method are relatively low it is labor intensive and difficult to control quality.
In U.S. Pat. No. 5,419,171, an improved isostatic bulge forming method is described for forming a metal tube. The bulge forming apparatus uses a fluid pressure chamber having a valved inlet and a valved outlet for entry and egress of the forming fluid. The forming assembly has mated tool halves that were retained in a fixturing tube. When the tube stock blank was inserted in the tool, top and bottom annular caps of the assembly formed a fluid tight seal between the walls of the blank and the inside of the retaining tube.
To form the blank under isostatic conditions, the chamber was filled with fluid. The fluid surrounded the entire forming assembly. The fluid in the chamber was pressurized and the tube bulged into the compressible air spaces or evacuated volume between the blank and the configured inner surface of the die. Once the blank was formed, the pressure on the fluid was relieved, the assembly was removed from the chamber, and the part was removed from the assembly. I follow essentially the same steps in the present invention with essentially the same basic equipment. I use, however, sliding plugs in the present invention to apply end loads to the blank, and achieve better performance and increased versatility without significantly higher tooling costs.
Because equal forces are applied in the isostatic bulge forming process to both sides of the tools in the chamber, there is no need for high strength tooling. The isostatic nature of the pressure application to an unrestrained assembly allows considerable latitude for bent and contorted tube shapes, so isostatic bulge forming is particularly suited for aerospace manufacturing.
As U.S. Pat. No. 5,419,171 (Bumgarner) illustrates, sealing between the tube stock blank and the forming die (actually the retaining cylinder) occurs along the wall of the blank with o-ring seals on the annular tooling end caps. The ends of the blank are free from engagement with tooling and extend beyond the length of the die. In the present, invention, sliding or floating plugs that are able to slide within the forming die apply compressive end loads to the blank during forming. These sliding plugs make the process more versatile. The improved bulge forming process of the present invention allows greater elongation of the blank than Bumgarner achieved because the end loading effectively feeds material in the blank to the die cavity during forming. Elongation can nearly be doubled. The specifications for corner radii can be set tighter.
U.S. Pat. No. 4,840,053 (Nakamura) describes a bulge forming method for manufacturing a pipe with projections, such as a manifold for automobiles. Bulges can be formed in sequential stages to assure adequate thickness in each branch pipe. Bulge spaces are filled with movably mounted spaces to facilitate the sequential forming. Holders at each end of the pipe are pressed axially inwardly to exert the bulging force on the pipe. In the present invention, sliding plugs are `self feeding` and move inwardly to compensate for the expansion during forming, thereby eliminating the need for hydraulic cylinders to press Nakamura 's holders inwardly.